Remote control or remote access systems use a transmitter and a receiver. An exemplary example is a garage door opener system where a transmitter is contained in a remote control unit and a receiver is connected to a garage door motor. The transmitter and receiver are typically comprised of different integrated circuits, although Texas Instruments Incorporated manufactures an integrated circuit having both transmission and reception capability. When activated, the transmitter sends a data pattern (usually a serial data stream modulated onto a radio frequency or infra-red carrier, to the receiver. The data pattern includes an identification sequence which is unique for each transmitter.
When the receiver receives and decodes a data pattern with the correct format, it extracts the identification sequence and compares it against one or more "authorized" sequences. If a match occurs, the transmitter is assumed to be legitimate and access is granted. However, there are instances wherein the transmitter is not ligitimate such as in the case where an eavsdropper may have obtained the identification code of the transmitter by monitoring data transmissions. It is thus desirable to prevent unauthorized learning and retransmission of a code.